Uniaxial Thin Films Research Articles

Determination of molecular arrangement in extremely uniaxial solid thin films of a linear bisazo dye

Abstract The molecular arrangement of the extremely uniaxial thin film was determined using X-ray analysis, including grazing incidence wide-angle X-ray scattering. The highly oriented film was obtained by depositing a bisazo dye onto an aligned polytetrafluoroethylene (PTFE) layer via vacuum evaporation, as shown previously. The X-ray analysis indicated that the molecules are arranged in parallel or antiparallel orientations within the unit cell. Moreover, their long axes are parallel to each other within the grains, which are uniformly oriented throughout the film. These results confirm the driving force of the orientation reported previously using a molecular dynamics model: dye molecules trapped along the atomic grooves between adjacent PTFE chains serve as nuclei for crystal growth. In addition, the long molecular axes remain parallel to the rubbing direction, although some grains are inclined in the short-axis direction. This molecular arrangement in the film could contribute to a high degree of uniaxial orientation.

Optical Properties and Optimization of LaB6 Thin Films for Photothermal Applications

AbstractThe growth of highly crystalline LaB6 films with an excellent optical response and low loss is reported, which will be useful for high‐performance photothermal device applications when combined with their inherent refractory properties. Optimum growth parameters for realizing uniaxial and coherent LaB6 thin films, exhibiting an excellent plasmonic response for near‐ to mid‐infrared device applications, are established. Numerical electromagnetic simulations of the epitaxial LaB6 nanostructures revealed that the electromagnetic field at the LaB6 surface can be as high as that of the Au nanostructures. Furthermore, the LaB6 nanostructures show resonance in the visible (red) to mid‐infrared region comparable to those of Au with the added advantage of improved temperature stability that can withstand harsh photothermal device operations.

Nondestructive Detection of Magnetic Contaminant in Aluminum Casting Using Thin Film Magnetic Sensor.

The thin film magneto-impedance sensor is useful for detecting a magnetic material nondestructively. The sensor made by single layer uniaxial amorphous thin film has a tolerance against surface normal magnetic field because of its demagnetizing force in the thickness direction. Our previous study proposed the sensitive driving circuit using 400 MHz high frequency current running through the sensor to detect the logarithmic amplifier. We also confirmed the sensitivity of the sensor within 0.3 T static normal magnetic field, which resulted in detection of 5 × 10−8 T of 5 Hz signal. This paper proposes a nondestructive inspection system for how detecting a contaminant of small tool steel chipping in aluminum casting specimen would be carried out. Three channel array sensors installed in the 30 mT static field detecting area were fabricated and experimentally showed a detection of low remanence magnetic contaminant in a bulk aluminum casing specimen.

Structure and Dielectric Properties of Anisotropic n-Alkyl Anilino Squaraine Thin Films

Solution-processed and thermally annealed thin films from a series of n-alkyl terminated anilino squaraines are systematically investigated regarding their structural and optical properties by means of X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). Their characteristic intense double-hump-shaped absorbance spectra consisting of coupled H-aggregate and intermolecular charge transfer (ICT) resonance bands make them appealing for fundamental light–matter interaction studies, and their environmental sustainability invites for consumer optoelectronic applications. Now, the single-crystal structure of the n-pentyl anilino squaraine (nPSQ) provides a missing link to identify potential odd–even effects with respect to the terminal alkyl chain for bulk crystals. While all single crystals adopt a triclinic unit cell with biaxial dielectric properties, the thin films condense into effectively uniaxial anisotropic thin films. Here, the inherently low sensitivity to the out-of-plane complex refractive index of SE in reflection is reasonably compensated by adding transmission SE and transmission intensity data. With that, a general picture of the structural and dielectric properties of n-alkyl anilino squaraine thin films is launched, revealing that the terminal alkyl chain length affects the structural and optical properties in a twofold way: we observe a steady change which is superimposed by an anisotropic odd–even effect.

Nanocombing Effect Leads to Nanowire-Based, in-Plane, Uniaxial Thin Films.

The fabrication of thin films comprising ordered nanowire assemblies with emerging, precisely defined properties and adjustable functionalities enables highly integrated technologies in the fields of microelectronics and micro system technology, as well as for efficient power generation, storage, and utilization. Shear force, theoretically, is deemed the most promising method for obtaining in-plane, uniaxial thin films comprising nanowires. The success depends largely on the assembly process, and uniform structural control throughout multiple length scales can be achieved only if a rational strategy is executed. Here, we report that in shearing processes dopants such as lyotropic cellulose nanorods can give rise to the uniaxial alignment of V2O5· nH2O nanowires. Our systematic study indicates that this finding, namely, the nanocombing effect, can be a general principle for the continuous production of uniaxial thin films comprising densely packed nanowires varying in chemical composition and aspect ratios. Conversion of the V2O5· nH2O constituents via in situ oxidative polymerization leads to in-plane, uniaxial polyaniline (PANI) thin films with anisotropic electric and optical properties, which are otherwise difficult to fabricate due to the poor processability of PANI. The uniaxial PANI thin films can be utilized to fabricate flexible gas sensors for distinguishing various analytes, including similar homologues such as methanol and ethanol. We expect the methodology to be applied to a broad spectrum of synthetic and biogenic nanowires for the integration of their collective properties in high-performance electronic devices.

Optical anisotropy in vertically oriented TiO2 nanotube arrays

Nanofabricated optically anisotropic uniaxial thin films with deep submicron feature sizes are emerging as potential platforms for low-loss all-dielectric metamaterials, and for Dyakonov surface wave-based subwavelength optical confinement and guiding at interfaces with isotropic media. In this context, we investigate the optical properties of one such uniaxial platform, namely self-organized titania nanotube arrays (TNTAs) grown by the bottom-up nanofabrication process of electrochemical anodization on silicon wafer substrates, and subsequently annealed at different temperatures, i.e. 500 °C and 750 °C. We performed detailed quantitative analysis of the structure of the TNTAs using x-ray diffraction and Raman spectroscopy, which revealed a measurable phonon confinement in TNTAs annealed at 500 °C. Variable angle spectroscopic ellipsometry was used to investigate the optical anisotropy in two kinds of TNTAs—those constituted by anatase-phase and those containing a mixture of anatase and rutile phases. Both kinds of TNTAs were found to have positive birefringence (Δn) exceeding 0.06 in the spectral region of interest while mixed phase TNTAs exhibited Δn as high as 0.15. The experimentally measured anisotropy in the refractive index of the TNTAs was compared with the predictions of two different effective medium approximations incorporating the uniaxial geometry. The measured value of Δn for TNTAs exceeded that of bulk anatase single crystals, indicating the potential of nanostructured dielectrics to outperform dielectric crystals of the same material with respect to the magnitude of the achievable directional refractive index contrast.

Design of Heterogeneous Nuclei for Lateral Crystallization via Uniaxial Assembly of Cellulose Nanocrystals

Semicrystalline polymers (SCPs) represent a group of cheap heterogeneous nuclei for crystallization. Nevertheless, cellulose, the most abundant biogenic SCP, is notorious for its poor processability. This limits its application as the orientational guiding agent in crystallization of functional compounds. Different from current polymer engineering approaches to uniaxial SCP thin films, we explored a novel approach to the uniaxial cellulose thin film via the oriented assembly of cellulose nanocrystals (CNCs) by means of a simple dip-coating technique. This thin film successfully guides the lateral crystallization of two drug compounds, which in turn reflects the uniformity of the uniaxial CNC alignment on the macroscopic scale. Furthermore, unlike traditional SCP thin films, the assembly route driven by different external forces can lead to CNC thin films with distinct orientational characters for fabrication of patterned drug thin films. The emerging colloidal assembly route to a uniaxial SCP substrate le...

Phenomenology of current-induced skyrmion motion in antiferromagnets

We study current-driven skyrmion motion in uniaxial thin film antiferromagnets in the presence of the Dzyaloshinskii–Moriya interactions and in an external magnetic field. We phenomenologically include relaxation and current-induced torques due to both spin–orbit coupling and spatially inhomogeneous magnetic textures in the equation for the Néel vector of the antiferromagnet. Using the collective coordinate approach we apply the theory to a two-dimensional antiferromagnetic skyrmion and estimate the skyrmion velocity under an applied DC electric current.

Critical behavior and universality properties of uniaxial ferromagnetic thin films in the presence of random magnetic fields

Critical phenomena in uniaxial ferromagnetic thin films in the presence of random magnetic fields have been studied within the framework of effective field theory. When the type of the random field distribution is bimodal, the system exhibits tricritical behavior. Furthermore, the critical value of surface to bulk ratio of exchange interactions at which the transition temperature becomes independent of film thickness varies very slowly in the presence of both bimodal or trimodal random fields. Regarding the universality properties, neither p, nor h variations in the system can affect the value of the shift exponent λ. In this regard, it can be concluded that pure ferromagnetic thin films are in the same universality class with those under the influence of random discrete magnetic fields.

Optical Anisotropy of Flagellin Layers: In Situ and Label-Free Measurement of Adsorbed Protein Orientation Using OWLS

The surface adsorption of the protein flagellin was followed in situ using optical waveguide lightmode spectroscopy (OWLS). Flagellin did not show significant adsorption on a hydrophilic waveguide, but very rapidly formed a dense monolayer on a hydrophobic (silanized) surface. The homogeneous and isotropic optical layer model, which has hitherto been generally applied in OWLS data interpretation for adsorbed protein films, failed to characterize the flagellin layer, but it could be successfully modeled as an uniaxial thin film. This anisotropic modeling revealed a significant positive birefringence in the layer, suggesting oriented protein adsorption. The adsorbed flagellin orientation was further evidenced by monitoring the surface adsorption of truncated flagellin variants, in which the terminal protein regions or the central (D3) domain were removed. Without the terminal regions the protein adsorption was much slower and the resulting films were significantly less birefringent, implying that intact flagellin adsorbs on the hydrophobic surface via its terminal regions.

Theory of magnetic domains in uniaxial thin films

For uniaxial thin films, properties of magnetic domains are usually described within the Kittel model, which assumes that domain walls are much thinner than the domains. In this work we present a simple model that includes a proper description of the magnetostatic energy of domains and domain walls and also takes into account the interaction between both surfaces of the film. Our model describes the behaviour of domain and wall widths as a function of film thickness, and is especially well suited for the strong-stripe phase. We prove the existence of a critical value of magneto-crystalline anisotropy above which stripe domains exist for any film thickness and justify our model by comparison with exact results. The model is in good agreement with experimental data for hcp cobalt.

Simulating the Emission Properties of Luminescent Dyes within One-Dimensional Uniaxial Liquid Crystal Microcavities

We have developed a simulation method which is capable of simulating the emissive properties of luminescent dyes inside uniaxial anistropic thin film microcavities[1]. The method uses plane wave decomposition of the electric field of a dipole antenna in a uniaxial medium and a scattering matrix formalism to account for interference and reflection from the various interfaces in the device. We apply this method to simulate the excitation of waveguide modes in a slab waveguide formed by reorientation of a liquid crystal. We investigate the emission both outside the device and into waveguided modes inside the liquid crystal device.

Magnetic anisotropy in epitaxial Mn5Ge3films

High crystalline quality Mn 5 Ge 3 films with thicknesses ranging 4–200 nm have been grown on Ge(111) substrates by solid phase epitaxy. The basal hexagonal plane of Mn 5 Ge 3 is in epitaxy with the Ge(111) plane. Magnetic properties of the films have been investigated as a function of the film thickness and the magnetization curves have been analyzed using a theory that includes a description of magnetic domains in uniaxial thin films. The results clearly indicate the existence of a critical thickness below which the magnetic stripe phase disappears. We have determined the value of this thickness to lie between 10 and 25 nm from the analysis of experimental magnetization curves and the theoretical fit of the in-plane remanent magnetization. Although analogies can be drawn between the behavior observed in our system and that of hcp Co, we have shown that the critical thickness is considerably smaller in Mn 5 Ge 3 ; this has the potential to open new fields of applications for Mn 5 Ge 3 thin films in magnetic recording and spintronics.

Role of dopants on the electro-optic effect in nematic liquid crystals aligned with doped polypyrrole

In order to estimate electro-optical response times, measurements of the transmitted light intensity through positive uniaxial nematic thin films in planar alignment have been performed. Very fast relaxation times were experimentally found for nematic samples aligned with conductive polymers, namely, polypyrrole doped with 5 types of anions having different molecular sizes and geometries. Using undoped polypyrrole alignment layers, slower electro-optical responses were obtained. Given that the improvement in the relaxation times is induced by the presence of the anions in the polypyrrole films, the increasing of the total restoring torque of the nematic molecules to the confining surfaces—responsible for the short relaxation times—is related to the charges accumulated at the nematic-conductive polymer interface.

Spin-transfer pulse switching: From the dynamic to the thermally activated regime

The effect of thermal fluctuations on spin-transfer switching has been studied for a broad range of time scales (subnanoseconds to seconds) in a model system, a uniaxial thin film nanomagnet. The nanomagnet is incorporated into a spin-valve nanopillar, which is subject to spin-polarized current pulses of variable amplitude and duration. Two physical regimes are clearly distinguished: a long pulse duration regime, in which reversal occurs by spin-transfer assisted thermal activation over an energy barrier, and a short-time large pulse amplitude regime, in which the switching probability is determined by the spin-angular momentum in the current pulse.

Landau-Lifshitz equation for magnetization dynamics of thin films: Failure of the linear approximation at low effective fields

We have obtained numerical solutions of the Landau-Lifshitz equation for the nonlinear response of a monodomain uniaxial ferromagnetic thin film subjected to static longitudinal and sinusoidal transverse in-plane magnetic fields with the easy axis exactly or nearly transverse to the static field. For transverse anisotropy, the well-known linear approximation predicts that the ferromagnetic resonance frequency goes to zero when the longitudinal field is equal to the anisotropy field, that is, when the static internal field goes to zero. This is not correct for nonzero sinusoidal fields. At low resonance frequency, the resonance field is less than the anisotropy field. This shift increases as the ac field is increased. The difference between the linear and nonlinear values of resonance frequency for a given static field is explained in terms of switching times shorter than the period at low frequency.

Thermodynamics of strained vanadium dioxide single crystals

Vanadium dioxide undergoes a metal–insulator transition, in which the strain condition plays an important role. To investigate the strain contribution, a phenomenological thermodynamic potential for the vanadium dioxide single crystal was constructed. The transformations under the uniaxial stress, wire, and thin film boundary conditions were analyzed, and the corresponding phase diagrams were constructed. The calculated phase diagrams agree well with existing experimental data, and show that the transformation temperature (and Curie temperature) strongly depends on the strain condition.

Magnetostatic bulk and surface spin-wave focusing in antiferromagnetic thin films

We study the focusing of bulk and surface spin waves in thin antiferromagnetic films. This study is carried out by solving the dispersion relation for the bulk and surface waves to obtain the constant frequency curves in $k$ space (slowness surfaces). The focusing pattern is obtained from the slowness surface by evaluating the curvature at each point on the surface. We have studied the focusing pattern in three uniaxial antiferromagnetic thin films, namely, ${\text{MnF}}_{2}$, ${\text{FeF}}_{2}$, and ${\text{GdAlO}}_{3}$. It is found that the bulk modes in the lower band are focused into a larger number of directions compared to the upper one. The focusing of the bulk modes is stronger when compared to the surface modes. The general trends in the focusing behavior are not affected by changes in anisotropy or exchange fields. However, changes in the frequency or the applied field alter the focusing pattern.

1304 光学異方性多層膜の構造設計に関する検討 : 物性値と膜厚の最適化について(OS7-1 設計と最適化I-構造最適化-,OS7 設計と最適化)

Optical multilayer thin films are used for many optical devices for generating various specific filter function. Design of high performance optical multilayer thin films and cost saving are expected by using anisotropic materials in the multi-layer structure. Uniaxial optical anisotropic multilayer thin films are optimized to satisfy the design specifications by genetic algorithms in this study. Optimization program is developed for simultaneous optimization of number of layers, film thickness, refractive index and direction of director in each film. Some results of optimum design are reported.

The new matrix and the polarization state of the transmitted light through the cholesteric liquid crystal

The cholesteric liquid crystal can be regarded as a multilayer that consists of many layer uniaxial thin films that exhibit optical rotation. For the convenient discussion of the polarization states of the transmitted wave through a cholesteric liquid crystal, a new 2×2 matrix is given, in which the matrix element is changed into the complex exponential. Here we discuss the treatment process of this method in detail and its role in the investigation of the polarization states. Using this method we easily study the polarization states of the transmitted wave. The calculated results show that the polarization states of wave will be changed when it propagates along the cholesteric liquid crystal.